Checked other resources

• This is a bug, not a usage question.
• I added a clear and descriptive title that summarizes this issue.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangGraph rather than my code.
• The bug is not resolved by updating to the latest stable version of LangGraph (or the specific integration package).
• This is not related to the langchain-community package.
• I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Related Issues / PRs

#7273

Reproduction Steps / Example Code (Python)

from langchain_core.messages import HumanMessage, AIMessage
from langgraph.graph.message import add_messages

# 1. Message IDs are lost
left = [HumanMessage(content="hello", id="msg-1")]
right = [AIMessage(content="world", id="msg-2")]

result = add_messages(left, right, format="langchain-openai")

print(result[0].id)  # None — expected "msg-1"
print(result[1].id)  # None — expected "msg-2"

# 2. Custom additional_kwargs are lost
msg = AIMessage(
    id="msg-1",
    content="hello",
    additional_kwargs={"widgets": [{"type": "carousel"}]},
)
result = add_messages([], [msg], format="langchain-openai")
print(result[0].additional_kwargs)  # {} — expected {"widgets": [{"type": "carousel"}]}

Error Message and Stack Trace (if applicable)

No error raised. Message IDs are silently set to None and custom additional_kwargs are silently dropped.

Description

add_messages(format="langchain-openai") loses message IDs and custom additional_kwargs during the OpenAI format round-trip.

_format_messages() in langgraph/graph/message.py calls convert_to_openai_messages() without include_id=True. The round-trip BaseMessage → OpenAI dict → BaseMessage strips all IDs to
None. Additionally, any custom additional_kwargs (e.g. UI widgets, metadata) are dropped because convert_to_openai_messages only outputs standard OpenAI fields.

Impact:

1. Every subsequent reducer call assigns new random UUIDs to all existing messages, breaking ID-based deduplication and update logic.
2. Custom data stored in additional_kwargs is silently lost after each reducer invocation.

Proposed fix: Pass include_id=True to preserve IDs. Save additional_kwargs by message ID before conversion and restore lost fields after.

System Info

System Information

OS: Darwin
OS Version: Darwin Kernel Version 25.3.0: Wed Jan 28 20:56:42 PST 2026; root:xnu-12377.91.3~2/RELEASE_ARM64_T8142
Python Version: 3.14.2 (main, Dec 9 2025, 19:29:30) [Clang 21.1.4 ]

Package Information

langchain_core: 1.2.20
langsmith: 0.6.4
langgraph_sdk: 0.3.12

Optional packages not installed

deepagents
deepagents-cli

Other Dependencies

httpx: 0.28.1
jsonpatch: 1.33
orjson: 3.11.6
packaging: 25.0
pydantic: 2.12.5
pytest: 9.0.2
pyyaml: 6.0.3
requests: 2.32.5
requests-toolbelt: 1.0.0
tenacity: 9.1.2
typing-extensions: 4.15.0
uuid-utils: 0.13.0
zstandard: 0.25.0